1. Field of the Invention
The present invention generally relates to a printing method of thermal printer, and more particularly to a printing method by which a printing quality of thermal printer can be improved.
2. Prior Art
FIG. 1 is a block diagram showing an electric constitution of conventional thermal printer In FIG. 1, designates a line buffer for storing print data DB which has been subjected to a dot conversion, and 2 designates a control section having a micro processing unit (MPU), a working memory and a program memory. This control section 2 has a function for reading out the print data DB stored in the line buffer 1 and another function for inputting control signals and data into several kinds of circuits which will be described later. In addition, 3 designates an interface circuit which executes the communication of data between the control section and an external device (not shown, e.g. a micro computer). Further, 4 designates a head consisting of a shift register circuit 5, a latch circuit 6, a driver circuit 7 and a heating body 8. The shift register circuit 5 is constituted by a serial-in-parallel-out shift register, and the shift register circuit 5 reads the print data DB outputted from the control section 2 based on a clock signal CLK and then outputs the read print data DB to the latch circuit 6. The latch circuit 6 reads the output of shift register circuit 5 based on a latch signal DR outputted from the control section 2 and then outputs the read output of shift register circuit 5 to the driver circuit 7. This driver circuit 7 consists of four blocks, i.e., four drivers a to 7d. The driver 7a consists of NAND gates Ga1 to Gan, the driver 7b consists of NAND gates Gb1 to Gbn, the driver 7c consists of NAND gates Gc1 to Gcn, and the driver 7d consists of NAND gates Gd1 to Gdn. Each of first input terminals of these NAND gates is connected to each output terminal of the latch circuit 6, while second input terminals of the NAND gates within each block (or each driver) are connected together in common. The heating body 8 consists of heating elements THa1 to THan, THb1 to THbn, THc1 to THcn and THd1 to THdn. Each of first terminals of these heating elements is connected to the output terminal of corresponding NAND gate within the drivers 7a to 7d, while second terminals of these heating elements are all connected in common to a positive power source +V.
Next, 9 designates a timer circuit. When the control section 2 supplies common pulses CM1 to CM4 to the timer circuit 9, the timer circuit 9 sequentially generates current-on pulse signals C1 to C4 each having a pulse width W1 corresponding to current-on data TD supplied from the control section 2. These current-on pulse signals C1 to C4 are sequentially generated by predetermined intervals. Each of these pulse signals C1 to C4 is outputted to the common connection point between the second input terminals of the NAND gates within each driver. In FIG. 1, 10 designates a motor drive circuit which drives a pulse motor (or a step motor) 11 by one pulse based on a control signal MC supplied from the control section 2. This pulse motor 11 revolves a platen roller 12.
In such thermal printer which is constituted as described heretofore, the control section 2 inputs the print data DB outputted from the external device via the interface circuit 3, and then the control section 2 stores the inputted print data DB in the line buffer 1. Next, the control section 2 supplies first print data DB(1) for printing a first print line to the shift register circuit 5 in synchronism with the clock signal CLK. In addition, the control section 2 supplies the current-on data TD to the timer circuit 9. When the first print data DB(1) has been stored in the shift register circuit 5, the control section 2 supplies the latch signal DR to the latch circuit 6 to thereby keep the first print data DB(1) in the shift register circuit 5. At the same time, the control section 2 supplies second print data DB(2) to the shift register circuit 5. Next, the control section 2 sequentially outputs the common pulses CM1 to CM4 to the timer circuit 9 by the predetermined intervals, so that the timer circuit 9 sequentially generates current-on pulses C11 to C41 show in FIG. 2. Each of these current-on pulses C11 to C41 is supplied to each common connection point of the NAND gates within each driver. Due to these current-on pulses C11 to C41, the output terminal of NAND gate whose first input terminal is at "1" level becomes "0" level. As a result, current flows through the heating element connected with the NAND gate whose output terminal is at "0" level. In this case, the area corresponding to the NAND gate whose output terminal is at "0" level is printed, but the area corresponding to the NAND gate whose output terminal is at "1" level is not printed. Thus, the printing of a first print line will be executed. After the printing of first print line is completed, the control section 2 drives the pulse motor 11 so as to transport a printing paper forward by one step. In this case, a period T1 shown in FIG. 2 designates a period between a first time when the control section 2 supplies the control signal MC to the motor drive circuit 10 and a second time when the pulse motor 11 actually starts to revolve and then completes revolution of one step.
Thereafter, the similar printing operation as described heretofore is repeatedly performed on the print data DB(2) to DB(N), so that printing of one page will be completed.
Next, description will be given with respect to the detailed printing process of thermal transfer type thermal printer, illustrated in FIG. 3.
In FIG. 3, a transfer ribbon 13 and a printing paper 14 piled together are inserted between the thermal head 4 and the platen roller 12. In this case, the heating element THa1 arranged at a center portion of the edge end of thermal head 4 presses the transfer ribbon 13. This heating element THa1 is heated in a printing mode so that ink painted on the transfer ribbon 13 will be melted and then the melted ink will be adhered to the printing paper 14. Thus, the thermal transfer is performed.
Meanwhile, in the case where the conventional thermal printer performs the printing when the surrounding temperature is relatively low, white lines (or spaces)are formed on the, printed paper in a print line direction as a result of a phenomenon, referred to as in which there appears or occurs white lines amongst the whole printed result.
FIG. 4 shows an example of printing which results when the sticking phenomenon occurs. In FIG. 4, a print line N+1 is shifted from a predetermined printing position and certain part thereof is printed over a print line N, so that interval portion between the print lines, N+1 and N+2 must become large. Therefore the resulting interval portion between print lines N+1 and N+2 is seen as a white line.
Next, description will be given with respect to the cause for occurring the sticking phenomenon. As described above, this sticking phenomenon occurs when the surrounding temperature about the thermal head 4 is low. The cause of sticking phenomenon will be as follows. When the surrounding temperature about the thermal head 4 is low, the control section 2 must widen the pulse widths of current-on pulse signals C1 to C4 in order to raise the heating temperature of each heating element of the thermal head 4, to predetermined printing temperature. On the other hand, in the case where the pulse widths of these pulse signals C1 to C4 are so widened, the heating elements which are supplied with the current-on pulses in initial orders must be cooled down. For this reason, after the ink on the surface of transfer ribbon is melted due to the heat of the heating element, the melted ink is cooled and then adhered to the heating element. Thus, when the printing paper 14 is driven by one step after the printing of one print line is completed, the printing paper can not be transported forward by a predetermined distance. As a result, the interval distances between the print lines will become irregular.
As described above, in the case where the surrounding temperature is low, the transfer ribbon is adhered to the heating elements of thermal head so that the printing paper can not be transported forward in normal manner. Hence, the conventional thermal printer suffers a problem in that the white lines are formed in the print direction of thermal head so that the whole printed result is seen as having white lines formed across the printed matter in the direction of the thermal head 4.